1. Field of the Invention
The field of the present invention is silicon carbide-reinforced light weight alloy composite materials, and more particularly, improvements of composite materials comprising a matrix of a light weight alloy and a reinforcing material consisting of at least one of a silicon carbide whisker and a silicon carbide grain.
2. Description of the Prior Art
There are such conventionally known composite materials made using an Al-Mg based alloy which is an aluminum alloy as a light weight alloy and using a silicon carbide whisker with SiO.sub.2 removed as a reinforcing material (see Japanese Patent Application Laid-open No. 538/86).
It is alleged that the reason why SiO.sub.2 contained in the silicon carbide is removed in the prior art is because SiO.sub.2 may preferentially react with Mg in the Al-Mg based alloy during compounding to produce an intermetallic compound of Mg.sub.2 Si which is segregated to cause a reduction in strength of the resulting composite material.
However, the present inventors have made various reviews and as a result, have cleared up the following fact.
If the SiO.sub.2 content is zero, the strength of the composite material is reduced, and variation in strength is produced. If the SiO.sub.2 content is of a predetermined value, a compounding effect appears. If the SiO.sub.2 exceeds the predetermined value, the compounding effect is lost. These phenomena may be produced even when an Al-Cu based alloy or an Al-Si based alloy is used as a matrix.
When these respects are taken into consideration, it can be safely said that the strength of the composite material is governed not only by the reaction of Mg in the matrix with SiO.sub.2 and the like, but also by the content of SiO.sub.2 and the like contained in the silicon carbide whisker.
It is also known to use an aluminum alloy containing Mg and Cu in order to improve the strength characteristic of the composite material (for example, see Japanese Patent Application Laid-open Nos. 279647/86 and 199740/87).
However, there is the following problem: When a composite material is produced using such aluminum alloy by utilizing a pressure casting process, cracks may be produced in a molded product and thus, a composite material for a practical use cannot be provided, because the filling of a molten metal into a reinforcing molded product made of a silicon carbide whisker or the like cannot be smoothly conducted.
Further, it is known to use a casting Al-Si based alloy as the aforesaid aluminum alloy. An eutectic crystal silicon in this Al-Si based alloy precipitates in the form of a needle crystal to cause a reduction in toughness of a matrix. For this reason, one element selected from Sb, Na and Sr is added to a molten metal during casting to effect an improving treatment of such alloy in order to provide a spherical eutectic crystal silicon.
When such improving treatment is conducted, the toughness of a simple Al-Si base alloy material is improved, on the one hand, and the tensile strength thereof is reduced, on the other hand. With a composite material made using this Al-Si based alloy as a matrix, a problem of reductions in both of toughness and tensile strength arises.
Furthermore, when the intermetallic compound of Mg.sub.2 Si is produced as described above, it promotes wearing of a tool during cutting of the resulting composite material and reduces the life of the tool, because the intermetallic compound has a high hardness. A cutting mechanism for the composite material cuts the matrix while falling off the reinforcing material such as the silicon carbide whisker and the like from the matrix by the tool, but when the aforesaid compound is in close contact with the reinforcing material, it exhibits an anchoring effect of retaining the reinforcing material in the matrix, resulting in a problem that not only the life of the tool is shortened, but also the cutting efficiency is reduced.
With such a composite material, when an improvement in wear resistance thereof is intended to be provided, it is a common practice to enhance the volume fraction (Vf) of the silicon carbide whisker.
There is spontaneously a limit for the enhancement of the volume fraction as described above when the falling property of a molten metal is taken into consideration. In addition, the cost of the composite material is increased with an increase in content of the silicon carbide whisker.
Further, there are such composite materials made using as a light weight alloy, Mg-Al based and Mg-Al-Zn based alloys which are magnesium alloys.
However, such magnesium alloys have a problem that they are poor in wettability to the silicon carbide whisker and the like, thereby providing a lower interfacial bond strength between the silicon carbide whisker and the matrix is lower, with the result that a sufficient reinforcing power of the silicon carbide whisker and the like is not obtained in the resulting composite material. Another problem is that an intermetallic compound of Mg.sub.2 Si is produced by reaction of SiO.sub.2 and Mg, as describe above.
Yet further, it is considered that the wear resistance of such a composite material depends upon the matrix. For this reason, a wear resistant magnesium alloy having a smaller content of the aforesaid corrosion promoting constituents is employed.
Even if a wear resistant magnesium alloy as described above is employed, however, the following problem arises: If the corrosion promoting constituents are contained in a content exceeding a predetermined level in the reinforcing material, an electrolytic corrosion occurring between the corrosion promoting constituents and the matrix is activated in a corrosive environ-ment due to the fact that the corrosion promoting constituents are difficult to solid-solubilize in the wear resistant magnesium alloy. As a result, the wear resistance of the resulting composite material is substantially degraded.